This invention relates generally to receivers, and more specifically to a double superheterodyne receiver capable of multichannel reception. The receiver according to the invention is particularly well adaptable for use in the form of a multichannel transceiver.
As is well known, a double superheterodyne receiver employs two frequency converters or mixers for dual heterodyning before final detection or demodulation to the original intelligence. For multichannel reception by this type of receivers, therefore, the output signal (the second intermediate frequency signal) from the second frequency converter is required to have a constant frequency of, for example, 455 kHz regardless of the frequency of the incoming modulated radio frequency signal. To this end there have heretofore been employed two crystal-controlled local oscillators which supply to the respective frequency converters the signals with frequencies selected in accordance with the incoming signal frequency (see FIG. 1 of the accompanying drawings).
This prior art configuration requires a large number of quartz crystals to permit the local oscillators to produce different frequency signals. When the double superheterodyne receiver of this type is adapted for a transceiver for 23-channel communication is the citizens' waveband, as many as ten quartz crystals are necessary in its receiver section, and another four in the transmitter section.
More recently a double superheterodyne receiver or transceiver has been developed in which one of the local oscillators is replaced by a frequency synthesizer based on the phase-locked loop (PLL) principle. The use of the PLL frequency synthesizer succeeds in a drastic decrease in the number of quartz crystals in use, as will be later explained in some more detail. According to the concepts of this invention, however, the number of the quartz crystals or of the crystal oscillators themselves is not yet reduced to an absolute minimum by the use of the PLL frequency synthesizer in the conventional way.